During the same period, ITV also saw its audience share among the same group decline by 18%.
ITV confirmed the figures and attributed the decline to the new series of 'Doctor Who' on BBC One and its own exceptional performance in the second week of April 2005, when 'Hell's Kitchen' proved popular with viewers in the 16- to 34-year-old bracket.
News of the dip coincides with a fall in ITV's share price. ITV's ad revenues are expected to be down by between 3% and 5% year on year in June, despite the Fifa World Cup. The TV market as a whole is expected to be down by between 5% and 8% in June.
A spokeswoman for ITV defended its performance in April by highlighting the popularity of its digital channels. She added that it had a slate of programmes aimed at 16- to 34-year-olds in the pipeline.
An insider at a rival broadcaster said the April dip was "bad news" for the TV industry as a whole and said ITV had suffered from not having scheduled "a big event like 'Hell's Kitchen' supported by extra 'Coronation Street'".
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